1. Field of the Invention
The present invention relates generally to laser drilling, and more particularly to a method and apparatus for drilling a hole in an article having a nonmetallic layer.
2. Description of the Related Art
Lasers are commonly used in the aerospace and power generation industries to drill holes in components. For example, lasers are commonly used to drill air cooling holes in turbine airfoils such as blades and vanes. A pulsed laser beam is directed at the article, and molten material from the focus region of the laser beam is expelled from the article. According to one known method, an Nd:YAG laser generates pulses having a pulse energy of 10 joules, a pulse duration of 1 millisecond, and a pulse repetition rate of 10 Hz, to drill holes in hollow turbine blades. The pulses are generated in a "free running" mode in which the lamps are pulsed at 10 Hz to produce the pulsed laser beam.
In aircraft engine and power generation equipment, thermal barrier coatings (TBCs), typically comprising a ceramic material, are often applied to the surfaces of components to provide thermal protection against the high temperatures which exist in operation. It has proven difficult, however, to manufacture holes in articles having a thermal barrier coating. For example, if the thermal barrier coating is applied after small diameter cooling holes have been drilled, the coating process often blocks the holes, since the total thickness of the coating, e.g. 0.010 inch, is typically of about the same magnitude as the hole diameter, e.g. 0.010-0.030 inch. Blocked holes are problematic, because they restrict or modify the air flow rate and pattern through the holes, which leads to poor engine performance.
On the other hand, performing the conventional laser drilling process after the thermal barrier coating has been applied to the article often generates cracks between the layers of coatings, leading to chipping or spallation of the coating, which reduces part life.
It would be desirable, therefore, to have a method and apparatus for drilling a hole through an article having a nonmetallic layer which would maintain the geometry of the hole and the integrity of the nonmetallic layer.